The present invention relates generally to the field of precision voltage references and, more specifically, to constant current sources which may be utilized as components of precision voltage references.
High stability, precision voltage references are critical requirements in electronic designs. Reference sources are needed that have extremely low values for temperature drift, noise, internal resistance and load sensitivity combined with a high degree of short and long term stability. Such voltage references may have a wide range of applications including precision comparators, power supplies and current sources as well as analog-to-digital and digital-to-analog converter references.
The most common reference elements are diodes built to break down at a characterized voltage. Most of these reference diodes break down in what is known as the avalanche region. For example, zener diodes are typically used in this application. The temperature coefficient of the voltage in the avalanche region is positive. For a low or zero temperature coefficient reference, a forward biased diode is usually wired in series with a reverse biased zener diode. The temperature coefficient of the forward biased diode is negative and can be constructed in a way as to exactly cancel out the positive temperature coefficient drift of the avalanche diode.
The effect of neutron damage on the avalanche breakdown diode is an increase in the bias voltage with the constant bias current. Conveniently, the effect on the forward biased diode from neutron damage is opposite that of the effect on the reverse biased element. Unfortunately however, the negative change is more than that which is needed and the net result is a negative drift. With proper gold doping of the forward biased diode, the negative charge can be reduced considerably. High stability devices have been demonstrated having near zero temperature coefficients from -55.degree. C. to 75.degree. C. with a reference shift of 10 mV after 3.times.10.sup.15 N/cm.sup.2 irradiation on a 6.2 V.sub.2 diode.
By way of example, a precision voltage reference is disclosed in the article entitled "A Radiation Tolerant, Programmable Precision Voltage Reference", co-authored by the present inventor and Art Peltier, appearing in the 1979 issue of the Journal of the IEEE Nuclear and Space Radiation Effects Conference. The voltage reference disclosed in that article is a monolithic, integrated voltage reference consisting of five sub-circuits and including a constant current source. Reference is made to that article which is hereby incorporated by reference. The aforesaid article illustrates a suitable application for the constant current source of the present invention.
It is known that a single junction field effect transistor can be used as a current source over a wide range of voltages with a relatively constant current when biased in its saturation region. The error of this type of current source is illustrated in FIG. 1 as an upward tilt in the current characteristic, i.e. the drain current plotted as a function of drain-to-source voltage, and is due mainly to the reduction of the effective channel length because of the increased depletion layer as the gate-drain voltage (V.sub.GD) increases.